1. Field of the Invention
The present invention relates to a circuit for resetting a magnetic field of a transformer used in a power converter, and more particularly, to a reset circuitry and method for effectively transmitting the magnetizing energy stored in a transformer core of a power converter and extending a switch duty cycle of the power converter.
2. Description of the Prior Art
Three conventional methods for resetting magnetizing energy of a transformer mainly include a tertiary winding reset circuit, an RCD reset circuit, and a resonant reset circuit. Please refer to FIG. 1 and FIG. 2. FIG. 1 is a circuit diagram of a tertiary winding reset circuit according to the prior art. FIG. 2 is a timing diagram of the circuit of FIG. 1. The tertiary winding reset circuit as shown in FIG. 1 includes a diode 17 connected in series with a tertiary winding of a transformer T. The transformer T has a magnetizing inductor 14 and a first switch 12 for controlling a duty cycle of the transformer T. In FIG. 2, a rectangular pulse 12 represents the signal voltage of the first switch 12, VS1 represents the voltage across the first switch 12, and IM represents the current flowing through the magnetizing inductor 14 of the transformer T. When the first switch 12 is turned on, the transformer T charges the magnetizing inductor 14 until the first switch 12 is turned off. At this moment, the diode 17 connected in series with a tertiary winding of the transformer T is turned on and the magnetizing inductor 14 discharges through such loop until the magnetizing energy thereof is fully discharged. The magnetizing inductor 14 will not conduct current until the next ON period of the first switch 12, as can be seen from the current waveform IM of FIG. 2. If the turn ratio of the primary winding and tertiary winding of the transformer T is 1:1, the charging time and discharging time of the magnetizing inductor 14 will be equal with each other. In other words, the magnetizing energy stored in the magnetizing inductor 14 has to be fully discharged before the first switch 12 is turned on again, and thus the maximum duty cycle of the transformer T is limited within 0.5. Assuming that the duty cycle of the first switch 12 is D=⅓, then IM only needs one-half of the time necessary to release the magnetizing energy during the OFF period of the first switch 12. When the magnetizing energy is fully discharged, the voltage drop across the primary winding of the transformer T is rated at zero, and the value of VS1 decreases from 2VIN to VIN. According to the above-mentioned, it is known that the most serious drawback of the tertiary winding reset circuit is that the duty cycle of the first switch 12 is limited to a maximum value of 0.5.
Please refer to FIG. 3 and FIG. 4. FIG. 3 is a circuit diagram of an RCD reset circuit according to the prior art. FIG. 4 is a timing diagram of the circuit of FIG. 3. For the purpose of giving a clear explanation, like elements have the same reference numerals in the drawings. The RCD reset circuit includes a resistor 19 connected in parallel with a capacitor 18, wherein both of them are then connected in series with a diode 17, and finally connected to a primary winding of the transformer T. When the first switch 12 is turned on, the transformer T charges the magnetizing inductor 14 until the first switch 12 is turned off. At this time, the diode 17 of the RCD reset circuit is turned on, and the magnetizing inductor 14 resets the internal magnetic field of the transformer T via the RCD reset circuit until the next ON period of the first switch 12. Because the parallel resistor 19 is a power-consuming element, when the RCD reset circuit resets the magnetizing energy of the transformer T, the resistor 19 will transform part of magnetizing energy into heat. It can be known from the foregoing that the most serious drawback of the RCD reset circuit is that the resistor 19 dissipates and transduces parts of the magnetizing energy into heat at the same time the inner magnetizing field of the transformer T is reset, and the dissipated energy cannot be retrieved. This reduces the efficiency of the transformer T.
Please refer to FIG. 5 and FIG. 6. FIG. 5 is a circuit diagram of a resonant reset circuit according to the prior art. FIG. 6 is a timing diagram of the circuit of FIG. 5. The resonant reset circuit includes a capacitor 18 connected in series with the resistor 19, wherein both of them are then connected in parallel with a primary winding of the transformer T. When the first switch 12 is turned on, the transformer T charges the magnetizing inductor 14 until the first switch 12 is turned off. The magnetizing current discharges through a loop comprised of the magnetizing inductor 14, the resistor 19, and the capacitor 18. This loop is referred to as a LC resonant loop. The operating efficiency of the transformer T is enhanced by resetting its internal magnetic via the LC resonant circuit, however the resonance caused by the LC resonant circuit will form a harmonic wave that causes an unexpected high voltage across the first switch 12, as indicated by a voltage VS1 shown in FIG. 6. According to the above-mentioned, the most serious drawback of the resonant reset circuit is that the harmonic wave caused by resonance forms an enormous high voltage across the first switch 12, so the transformer T requires a switch having a strong durability and resistivity against a high voltage as the first switch 12.
It is therefore a primary objective of the claimed invention to provide a circuitry for resetting a magnetic field in a transformer of a power converter to solve the above-mentioned problems.
According to the claimed invention, there is provided a reset circuitry for resetting an internal magnetic field of a transformer of a power converter when a main switch of the power converter is turned off. The reset circuitry includes a first capacitor connected in series with a winding of the transformer, a rectifier connected in parallel with a series circuit comprising the first capacitor and the winding of the transformer, a second capacitor, an auxiliary switch connected in series with the second capacitor to form a series circuit to be connected in parallel with the rectifier, and a switch control circuit. The switch control circuit is configured to turn on the main switch and turn off the auxiliary switch instantaneously to release the magnetizing energy in a magnetizing inductor of the transformer, and then store the magnetizing energy stored in the magnetizing inductor and charge the first capacitor, and turn on the auxiliary switch and turn off the main switch to transmit the magnetizing energy stored in the magnetizing inductor and the energy stored in the first capacitor to the second capacitor after a short period of time, and charge the magnetizing inductor and the first capacitor by the second capacitor after the magnetizing energy stored in the magnetizing inductor is fully discharged, thereby reset an internal magnetic field of the transformer.
The claimed invention has a general form in which a power converter includes main switch, and a transformer having at least a primary winding and a secondary winding. The primary winding is connected in series with the main switch for receiving a DC voltage and induce an AC voltage on the secondary winding according to an on/off state of the main switch, a switch control circuit, and a reset circuitry for resetting a magnetic field in the transformer. In addition, the transformer has a magnetizing inductor. The switch control circuit can turn on the main switch and turn off the reset circuit in response to a pulse signal generated therefrom to store magnetizing energy in the magnetizing inductor. The switch control circuit can turn on the reset circuit and turn off the main switch in response to another pulse signal generated therefrom to transmit the magnetizing energy in the magnetizing inductor to the reset circuitry after a short period of time.
These and other objectives of the claimed invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.